<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <nav id="menu">
        <ul>
          <li >
            <a href="#" data-id = '1'>菜单1</a>
          </li>
          <li>
            <a href="#" data-id = '2'>菜单2</a>
          </li>
          <li>
            <a href="#" data-id = '3'>菜单3</a>
          </li>
        </ul>
  
        <div>
          <ul style="display: none">
            <li><a href="#">子菜单1-1</a></li>
            <li><a href="#">子菜单1-2</a></li>
          </ul>
          <ul style="display: none">
            <li><a href="#">子菜单2-1</a></li>
            <li><a href="#">子菜单2-2</a></li>
          </ul>
          <ul style="display: none">
            <li><a href="#">子菜单3-1</a></li>
            <li><a href="#">子菜单3-2</a></li>
          </ul>
        </div>
      </nav>
      <script>
        const menu =document.querySelector('#menu')
    
        menu.addEventListener('click', function(e) {
           

          if(e.target.tagName === 'A') {
             
           const index = e.target.dataset.id

           const ul = document.querySelector(`div ul:nth-child(${index})`)
            
           ul.style.display = ul.style.display === 'none' ? 'block':'none'
          } 
        })
      </script>
</body>
</html>